In a capacitive touch panel device, a value (capacitance value) measured when a detection target such as a finger does not contact (touch) a touch panel is stored in advance as a baseline. In detecting the target, a difference between a value measured at the time of the detection and the baseline is used. This difference becomes close to zero if the detection target does not contact the touch panel, and becomes a large positive value if the detection target contacts the touch panel. Thus, a touch on the touch panel by the detection target can be detected based on the difference between the measured value and the baseline. However, a value measured when the detection target touches the touch panel might be employed as the baseline. In this case, the generated baseline is not an appropriate baseline, so that a touch cannot be detected correctly. In response, an abnormal state caused by the inappropriate baseline should be detected and the baseline should be corrected.
According to an example of a responsive method, if a state where the aforementioned difference does not exceed a threshold (negative value) for determination about abnormality lasts for a certain period of time or more, it is determined that the baseline is in an abnormal state. If the baseline is determined to be in an abnormal state, the baseline is updated to be in a normal state (see Japanese Patent Application Laid-Open No. 2012-150747, for example). According to the method of Japanese Patent Application Laid-Open No. 2012-150747, if an object contacts a touch panel at the time of generation of a baseline and the object disappears (comes off the touch panel) thereafter, for example, this state is regarded as an abnormal state. In this case, a capacitance determined when the object disappears becomes lower than that determined when the baseline is generated, so that the aforementioned difference becomes a negative value to be used for detection.
A differential detection system is known as a system to enhance noise tolerance of a touch panel device (see Japanese Patent Application Laid-Open Nos. 2010-282539 and 2012-69083, for example). This detection system measures a difference in capacitance between a plurality of sensors arranged on a touch panel, and makes use of differences in capacitance between adjacent sensors for detection of multiple points (a plurality of touched positions).
A touch panel device employing a differential capacitance detection system having high noise tolerance such as that disclosed in Japanese Patent Application Laid-Open No. 2010-282539 or 2012-69083 cannot make the determination about an abnormal state and the baseline correction of Japanese Patent Application Laid-Open No. 2012-150747. This is due to the fact that the differential capacitance detection system cannot measure an absolute capacitance value so cannot recognize a capacitance reduction of a sensor. As an example, in response to change of a measured value corresponding to a difference in capacitance between sensors, this system finds difficulty in determining if the capacitance value of one of the sensors targeted for the measurement has increased or that of the other sensor has decreased. This determination becomes more difficult, particularly if adjacent sensors arranged in proximity are used in combination for measurement of a difference in capacitance therebetween, for example, as a finger having touched one of the sensors affects the other sensor.